The invention relates to an apparatus for X-ray analysis, comprising a source for X-rays to be analyzed, an analysis crystal for wavelength-analysis of the X-rays to be analyzed, and a detector for detecting the X-rays emanating from the analysis crystal, which analysis crystal is displaceable relative to the source along a first straight line which passes through the source, during said displacement the analysis crystal remaining tangent to a Rowland circle which extends through the source of X-rays to be analyzed and has a fixed dimension which is determined by the crystal geometry.
An X-ray analysis apparatus of this kind is known from U.S. Pat. No. 3,123,710. The apparatus described in the cited Patent, notably with reference to FIG. 1, therein is arranged to analyze fluorescence radiation emanating from a specimen to be examined. The fluorescence radiation is generated in the specimen by exposing the specimen to X-rays from an X-ray tube. Via a collimation slit (the entrance slit) the fluorescence radiation is directed onto the analysis crystal, after which the radiation originating from the crystal is collected by a detector with a collimation slit (the exit slit).
Because knowledge of the intensity distribution of the fluorescence radiation as a function of the wavelength is desired upon use of such an apparatus, the fluorescence radiation is wavelength-analyzed by the analysis crystal. This analysis is based on the well-known Bragg relation: 2dxc2x7sinxcex8=nxcex, in which d is the distance between the X-ray reflecting faces in the analysis crystal, xcex8 is the angle at which the radiation to be analyzed is incident on the analysis crystal, and xcex is the wavelength of the reflected radiation. This formula shows that the wavelength composition of the radiation to be analyzed can be determined by determining the intensity as a function of the angle of incidence and exit xcex8. This is realized by displacing and rotating the analysis crystal and the detector in such a manner that all angles xcex8 are traversed.
With a view to obtaining a suitable resolution and sensitivity, i.e. a suitable precision of measurement, the known apparatus is provided with a so-called focusing optical system, which in this case means that the entrance slit is imaged on the exit slit by the analysis crystal. This imaging effect is obtained by imparting a curved shape to the analysis crystal, for example by way of a spherical or circular-cylindrical surface, so that the analysis crystal has an imaging function in addition to an analyzing function. In order to ensure that the focusing condition is still satisfied, the entrance slit as well as the analysis crystal and the exit slit (i.e. the detector) must remain situated on a given circle, the so-called Rowland circle, during the traversing of all values of xcex8. The diameter of this Rowland circle is determined by the radius of curvature of the analysis crystal, so that it is constant during the measurement. The position of the Rowland circle is variable during the measurement, although the fixed entrance slit and the analysis crystal should always be tangent to the Rowland circle. During execution of the measurement (a so-called xcex8 scan) the analysis crystal is made to travel along a straight line, it being rotated in such a manner that it continues to be tangent to the Rowland circle. This straight line extends through the entrance slit which is considered to be the source of the X-rays to be analyzed for these measurements. (This is possible in that in principle no correlation exists in practice between the location and the direction of the X-rays at the area of the entrance slit as is also the case for a physical source). The detector should then follow a complex, leaf-shaped path which is commonly described as a lemniscate which should be very accurately followed. This requires a complex displacement mechanism which should satisfy very severe requirements in respect of precision and reproducibility.
A first problem is encountered if instead of carrying out a xcex8 scan it is desired to observe only a single wavelength (for example, in order to determine the intensity distribution in a characteristic X-ray line). Generally speaking, in such a case no displacement mechanism is required for the detector and the analysis crystal, because the detector and the analysis crystal can in principle remain in a fixed position. This is referred to as a so-called fixed measuring channel which, evidently, can be substantially less expensive than a variable measuring channel for a xcex8 scan.
In a spectrometer comprising a fixed measuring channel, the analysis crystal can thus in principle have a fixed position and orientation, but the problem then encountered is that often the orientation of the analysis crystal needs to be readjusted. Such readjustment must be performed because of the mechanical tolerances involved in the manufacture of the mounting frame and the holder for the crystal, and also because of thickness variations in the crystal itself, which variations would cause the reflected beam to be incident in the wrong location (i.e. not on the exit slit for the detector). Readjustment, moreover, is particularly necessary in the case of operation with X-rays of long wavelength, such as characteristic radiation of long light elements such as, for example borium having a characteristic radiation of a wavelength of 67 xc3x85E. In the case of such long wavelengths it is not possible to use a natural crystal for the analysis crystal, because the value d (the distance between the lattice faces in the crystal lattice) of such natural crystals is not of the required order of magnitude. In that case known so-called multilayer mirrors are used for wavelength analysis. A known drawback of multilayers consists in that even though the period of the layers of these mirrors is of the desired order of magnitude, it exhibits a large spread between the individual multilayer mirrors, for example of the order of magnitude of 4%. This means that a deviation of the same order of magnitude could also occur in the value of xcex8, which deviation, in conformity with said Bragg relation, must be compensated by a corresponding variation of xcex8, i.e. by rotation of the analysis crystal. Rotation of the analysis crystal, however, would rotate the direction of the emerging beam (i.e. the X-ray beam extending from the analysis crystal in the direction of the detector) through twice the correction angle, so that the (focused) beam would no longer reach the exit slit. If it were attempted to solve this problem by widening the exit slit, radiation would then be incident outside the actual detector material and/or too much background radiation would be collected by the detector, so that the measuring accuracy of the intensity measurements would be degraded inter alia because of a degraded signal-to-noise ratio and resolution.
It is an object of the invention to provide a solution to the problem of readjustment of the analysis crystal.
In conformity with a first aspect of the invention this problem is solved in that the apparatus comprises a fixed measuring channel in which the detector occupies a fixed position relative to the source during the measurement.
Because it is elected to move the crystal according to the known Rowland geometry during the readjustment (a motion which can be comparatively simply realized), which means that the crystal also performs a translatory motion during rotation, this motion will suffice if the angle xcex8 is not too large. In conformity with the theory of the focusing X-ray optics the detector should follow a piece of the path of the complicated lemniscate shape during readjustment, but the invention is based on the idea that this path can be approximated by a straight line for as long as the angles xcex8 are not too large. Thus, if the detector is made to stand still on this line-shaped part, said motion of the analysis crystal will cause only a displacement of the focus of the X-ray beam perpendicularly to the exit slit, but not noticeably in the transverse direction. Consequently, outside the focus only slight limited widening of the beam will occur; such widening is small in comparison with the total beam cross-section, because the imaging faults of the analysis crystal have already imparted a width to the beam which cannot be ignored.
Another problem occurs when a fixed measuring channel is desired for each of a number of different chemical elements (so different discrete wavelengths). In that case the comparatively complex displacement of the detector would again be necessary.
It is an object of the invention to provide a solution to this problem by providing a comparatively simple apparatus which comprises one fixed measuring channel which can be adjusted for various wavelengths.
In an apparatus of this kind the detector is displaceable with respect to the source; in conformity with a second aspect of the invention the apparatus comprises guide means for guiding the displacement of the detector along a second straight line.
Like the first aspect of the invention, the second aspect of the invention is based on the idea that this path can be approximated by a straight line, provided that the angles xcex8 are not too large. Because the implementation of a play-free and exact straight guide motion is easier (and hence less expensive) than a prescribed curved guide motion, a much simpler apparatus is obtained.
An embodiment of the apparatus in accordance with the invention is characterized in that the analysis crystal is formed by a multilayer mirror. The spread in respect of the layer thicknesses of such a mirror in such an apparatus does not seriously affect the performance of the apparatus, mainly because it is arranged to compensate deviations in this respect.
A further embodiment of the apparatus in accordance with the invention is characterized in that the angle enclosed by the source, the analysis crystal and the detector is larger than 120xc2x0. In this case the value of xcex8 is less than 30xc2x0 (i.e. xc2xd(180-120xc2x0)) and practice and calculations have demonstrated that the rectilinear approximation of the lemniscate path does not cause inadmissible deviations in this range.
Another embodiment yet of the apparatus in accordance with the invention is characterized in that the apparatus comprises a guide for moving therealong a point of the analysis crystal during displacement of the analysis crystal, which guide extends along a straight line passing through the source. This embodiment offers an inexpensive, play-free and accurate construction for guiding the analysis crystal along a path in conformity with the Rowland geometry.
Another embodiment of the apparatus in accordance with the invention is characterized in that the analysis crystal can be exchanged. Generally speaking, when another wavelength range of the apparatus is to be selected, a new analysis crystal will be required. When the analysis crystal is moved by means of a guide, during the manufacture of the carrier for the crystal only a different position of, for example, a guide cam cooperating with the guide need then be selected at the same setting of the relevant machine (for example, a numerically controlled milling machine). This cam can then be accurately positioned in a simple way. Even though transverse displacement of the detector will then usually be required so as to position it in the new range, the motion of the detector during the measurement will not be more complex. The transverse displacement can also be realized by inserting, for example a fixed spacer between the guide and the frame of the apparatus.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.